The invention relates to a color electrophotographic copying machine.
The color electrophotography which is utilized for copying purposes can be categorized into two transfer types, in which one employs the transfer of a toner image onto a record sheet, and the other transfers an electrostatic latent image. In the former technique, a plurality of color filters are used to separate a color original into a plurality of color separation images, as each individual color separation image is exposed to a photosensitive member, the resulting electrostatic latent image is color developed with a toner having a complementary color relationship with the color separation image to be developed, and subsequently the toner image obtained is transferred onto a record sheet. By superimposing a plurality of complementary color toner images which result from the individual color separation images on a single record sheet, a color electrophotographic image corresponding to the original is formed on the record sheet. In the latter technique, a color electrophotographic image corresponding to the original is formed on the record sheet by repeating the process of transferring an electrostatic latent image onto the record sheet as it is formed on a photosensitive member in response to each individual color separation image or directly forming an electrostatic laten image latent the record sheet which has a photosensitive layer, and then color developing the latent image on the record sheet.
In either technique, one of the most important problems is the registration of images which are to be superimposed one above another. Thus, during the process of obtaining a single color electrophotograph, there is the need to transfer either latent image or toner image onto the record sheet a plurality of times, and if there is any misalignment between the toner image or the latent image to be transferred and the record sheet during each transfer step, there results a color misalignment in the electrophotographic image obtained.
More particularly, such misalignment, which refers herein to the order of resolution of the naked eye, namely, 0.1 mm, is in two parts; i.e., a longitudinal misalignment which is attributable to the improper timing with which the record sheet, after having received a color image and moved in a particular direction for the purpose of developing or drying or both and returned to the original position, again meets with the photosensitive member or optical system which supplies another color image to the record sheet; and a lateral misalignment or misalignment in a direction transverse to the longitudinal misalignment which may be caused by uncontrollable tendencies of the sheet carrying system or other machine components. The longitudinal misalignment can essentially be overcome by improving the accuracy of a synchronizing apparatus which controls the timing with which the record sheet meets with the photosensitive member or optical system, but the lateral misalignment cannot be simply overcome because of the uncontrollable factors involved.
Another problem in color electrophotography accrues from the use of a plurality of developers of different colors during the developing process. Taking a conventional copying machine by way of example, the process of forming an electrostatic latent image on the surface of a photosensitive drum which rotates in one direction and then developing the latent image by direct contact with a developing solution during one revolution of the drum, is repeated during successive cycles, using developing solutions of different colors. However, with this process, the period of time for contact between the latent image and the developing solution may be too short to provide a satisfactory developing, or difficulty may be experienced when squeezing residual developing solution from the drum surface. Of greater importance is the effect of any remaining small amount of toner on the drum surface which may be mixed with a developing solution of different color to cause a discoloration.